Method of preparing a zirconium oxide that is crystallized mostly or completely in cubic form

ABSTRACT

1. A PROCESS FOR CONVERING MONOCLINIC ZIRCONIUM OXIDE TO CUBIC ZIRCONIUM OXIDE, WHICH MONOLINIC ZIICONIUM OXIDE HAS BEEN OBTAINED BY THE FLOTATION SEPARATION OF A ZIRCONIUM OXIDE-SILICON OXIDE MIXTURE, INCLUDING FINELY GRANULATING THE ZIRCONIUM OXIDE RECOVERED FROM SAID FLOTATION SEPARATION; TREATING SAID GRANULATE FROM SAID FLUORIIC ACID SOLUTION HAVING A CONCENTRATION OF BETWEEN 5 AND 10% SEPARATING THE SO-TREATED GRANULATE FROM SAID SOLUTION, MIXING SAID GRANULATE WITH AT LEAST ONE OXIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OF CALCIUM, MAGNESIUM, YTTRIUM, THORIUM AND CERIUM; AND HEATING SUCH MIXTURE TO ABOUT 1,000 TO 2,000:C., WHEREBY CONVERTING SAID MONOCLINIC CRYSTAL STRUCTURE SUBSTANTIALLY TO A CUBIC CRYSTAL STRUCTURE IN THE SOLID STATE.

United States Patent METHOD OF PREPARING A ZIRCONIUM OXIDE THAT ISCRYSTALLIZED MOSTLY R COM- PLETELY IN CUBIC FORM Klaus Deneke,Troisdorf-Lulsdorf, and Wilhelm Falkenhain and Peter Hack,Troisdorf-Ranzel, Germany, assignors to Dynamit NobelAktiengesellschaft, Troisdorf, Germany No Drawing. Continuation ofabandoned application Ser. No. 16,968, Mar. 6, 1970. This applicationJan. 31, 1972, Ser. No. 222,405

Int. Cl. C01g 25/02 US. Cl. 423-82 9 Claims ABSTRACT OF THE DISCLOSUREImprovements in preparing cubic zirconium oxide by separating mixedzirconia and silica by floatation, granulating the recovered zirconia,treating such with hydrofluoric acid solution, mixing this product withcalcium oxide or the like and then heating such mixture to about 1000 to2000 C. in order to convert the monoclinic crystal structure of thezirconia to a cubic crystal structure in the solid state.

This is a continuation of application Ser. No. 16,968 filed Mar. 6,1970, now abandoned.

It is in the prior art to prepare so-called stabilized zirconium oxide,i.e. zirconium oxide which is completely or substantially complete incubic crystal form, by melting suitable zirconium containing ores, suchas zirconium sand, Baddeleyite or zircite, in an electric arc furnacewith the required amount of carbon together with calcium oxide and/ormagnesium oxide as stabilizing oxides. This process reduces theundesired oxides that accompany the zirconium ores, usually SiO Fe O andTiO to metal. The high content of Si0 requires very large amounts ofelectric power for this reduction and therefore there is an economicdetriment.

Furthermore, the reduced metal alloy content is adjusted with theaddition of metallic iron, usually in the form of iron chips. Thespecific gravity of the alloy is thereby increased, making it possiblefor the alloy to sink into the bottom part of the melt. The magneticproperties of the alloy are improved by the addition of this iron,making it practical to remove the product of the following refiningprocess by magnetic separation.

This entire known process, however, is hard to control. The reduction ofthe ores which have a zirconium oxide content of 65 to 80% and a maximumpermissible SiO content of 0.5% for stabilized zirconium oxide isdifficult. The reduced alloy must be removable entirely from theproduct. The crushed material so produced must be oxidized at atemperature of at least 1300 C. In spite of all the difficulties andexpense, however, it is still very difficult to achieve a uniform endproduct that is the very delicate stabilized zirconium oxide (seeAmerican Pat. 2,535,526) of suitable quality.

According to another known process, zirconium ores are mixed withbasically reacting oxides, which have to be in a certain predeterminedratio to acidly reacting oxides, and the mixture is fused at about 1800C. The fused material is then crushed and finely ground, and thenmechanically separated into a coarser part consisting substantially ofzirconium oxide and a finer part consisting of silicates. The coarserportion is then treated with hydrochloric acid to destroy the silicatecontent, and then treated with caustic soda solution to dissolve thesilica gel that has been formed, the product is then again screened,washed and dried.

This known process is also complicated and expensive,

and difficult to control. Since its inception, it has been 3,849,532Patented Nov. 19, 1974 modified to use a mixture of Zirconium ores anddolomite, and eventually fluorspar too, with the temperatures usedranging between 1350 and 1450 C. The rest of the procedure has remainedas set forth above, i.e., the partially, fused mass is finelycomminuted, and a thin slurry thereof An attempt has also been made tosubstantially remove' the silica with hydrofluoric acid, but theprocedure starts out directly from fused zirconium ore, and requires agreat quantity of relatively highly concentrated acid. This last abovereferred to process is not intent upon preparing stabilized zirconiumoxide; cf. German Pat. 647,918.

Stabilized zirconium oxide is constantly growing in importance andusage, the properties of such material have become more stringentlyprescribed. While it may be desirable to use completely stabilizedzirconium oxide, it is often possible to use mixtures of monoclinic andcubic zirconium oxide even though these mixtures are 'much lesssensitive to thermal shock than the cubic structure. Most frequently,cubic zirconium oxide stabilized with C210 is required. Increasingly,however, the cubic structure stabilized with other oxides is demanded..These requirements' make it necessary to prepare product in batchesthat are chemically and physically very different,

and small in size in some cases. These widely varyingwashed and driedmaterial with 3 to 6 wt.-percent of finely granular calcium oxide orequimolar amounts-of other finely granular polyvalent metal oxidesuwhichcrystallize in cubic form only whose ionvradius is of the same orderof-magnitude as that .of the zirconium ion; and heating the mixturesthus obtained-in the form of molded pieces if desired-eat temperaturesbetween about 1000 and 2000 C., preferably between 1400 aiid l 900 C.,until the desired degree of transformation,otf ihecubic crystal form isachieved in a solid state reaction. This heating is for about2 to 30hours.

The oxides, for example of calcium, magnesium, ytrium, thorium, cerium,or mixtures of such oxides, can be used as the oxides in the process;instead of these metal oxides, other, metal compounds can be used whichyield the desired metal oxides by decomposition during...

the process. Examples of these other compounds are the hydroxides,carbonates, bicarbonates,.oxalates,-acetates,

or the like.

'The process of the invention thusconsists ofia plu rality,

of entirely separate and easily controlled individualsteps.

Thefirst step'utilizesthe process of W. German Green Pat. DAS 1,118,178.In this process zirconium sand is I fused in an arc furnace to decomposethe molten material to zirconium oxide and S102. The fused material isground 1 to a fine state of subdivision and then the two phases thus.formed are separated a flotation process; A monocllinic' zirconium oxideis obtained having a residualfcoritent pf about 0.65% SiQ The secondstep "of this proc ess further'refirie s intermediate monocliniczirconium oxide product. to a residual content of about0.2% SiO Thissecond step is performed.

with the use of hydrofluoric acid at room temperature. A relativelybrief treatment of about 1 to 3 hours with a 5 to aqueous hydrofluoricacid solution is surprisingly entirely sufficient. More than 75% of thedilute acid product is recovered and recycled back into the process. Thepurified zirconium oxide is decanted and dried.

In a third step of the process, the material produced by the second stepis then transformed substantially or completely from the monoclinic tothe cubic modification. This is not done, however, in a molten bath ashas been the practice hitherto, but in a solid reaction.

Very large amounts of stabilized zirconium oxide are needed inparticulate form having a high degree of fineness, usually up to about70 millimicrons. Grinding material which has already been fused andstabilized to this fineness, however, usually causes considerablecontamination by the grinding tools or by the silica content. Accordingto the process that is the subject of this invention, the materialproduced by the first and second steps of the procedure, that ismonoclinic form zirconium oxide of high purity and suflicient finenessis mixed with the oxides required for stabilization-cg, CaO and is thentransformed partially or completely to the cubic form at temperatures atwhich no more than a slight sintering occurs, e.g. about 1000 to 2000C., so that at most only a light crushing of the reacted product may bedesired or required.

If coarser material is required, the floated, fine grained monocliniczirconium oxide, which has been treated with hydrofluoric acid and mixedwith the necessary percentage of stabilizing oxides, is formed, by lightpressure, into suitably sized shaped articles, using an organic binderthat is volatile at low temperatures. If these bodies are heated forseveral hours at temperatures of, for example, 1850 C., sinteringoccurs. These bodies are then ground to obtain the desired coarser gritswhich have a compressive strength that makes them suitable for theirintended use.

The following Examples are illustrative of this invention.

EXAMPLE 1 150 kg. of a monoclinic zirconium oxide, purified according toDAS 1,118,178 down to 0.65% SiO is mixed with 150 liters of 8%hydrofluoric acid. This mixture is placed in a plastic tank that isresistant to hydrofluoric acid, and slowly agitated at room temperaturefor about 2 hours. Then the hydrofluoric acid is poured off; it can bereused. The resultant zirconium oxide is purified by repeated washingwith salt-free water and decanting whereupon it is dried. The Si0content of the material thus 0 treated has been reduced to 0.19%. Thisproduct is a fine grained powder having a size up to about 40millimicrons. 200 kg. of the monoclinic zirconium oxide, purified downto 0.19% SiO are mixed with 9.2 kg. of burnt lime of high fineness.Ethanol is added to the mixture as a binding agent, and then, underlight pressure, pieces are formed measuring about 100 x 200 x 30 mm.These pieces are then fired in a tunnel furnace with the temperaturetherein raised to 1790 C., from room temperature over a period of about24 hours. The firing time at this maximum temperature was about 6 hours,and the pieces leave the furnace at about 80 C. The resultant product ishardsintered pieces which are broken up, ground and then screened. Thegrinding resulted in a product having the following grain sizes insubstantially equal parts:

0-0.12 mm. 0.25-0.50 mm. 0.12-0.25 m. 0.50-1 mm.

These particles are sufliciently strong so that they can be molded intoshaped articles. X-ray examination shows that over 90% of the grainshave been transformed by the heat treatment from the monoclinic to thecubic crystal structure.

Test specimens of 10 x 10 mm. cross section and 50 mm. length werepressed from these particles with the use of 5% wax binder. Thesespecimens were heated to 1200 C. in a dilatometer. The measurementsshow, as reflected in the following table, that the thermal expansion ofthe product is completely uniform, returning to 0 after the coolingthereof.

Percent expansion Chemical analysis of the test showed, in addition toZrO the following values:

Percent A1 0 0.26 113F 8 0.041 g 0.06 TiO 0.062 CaO 4.30 Si0 0.21

EXAMPLE 2 In each case, 5 kg. of monoclinic, refined zirconium oxidehaving a grain size of up to 70 millimicrons, were mixed with fine limedust as in Example 1. A section of pipe with a diameter of mm. wasfilled with the mixture and heated in a gas-fired furnace. The firingtemperatures, firing time and contents of cubic zirconium oxide were:

Specimen 1:

4 hours 61% cubic material Specimen 2:

24 hours 92% cubic material.

The dilatometer measurements performed on all specimens produced theexpected thermal expansion curve similar to that shown in Example 1above.

In spite of the high percentage of cubic zirconium oxide in Specimen 1,it still clearly shows the influence of the monoclinic modification,i.e., in the 1000 to 1100 C. temperature range it still shows a phasetransformation to the tetragonal structure, with a correspondingcontraction in volume, depending on the percentage of monocliniczirconium oxide.

Specimen 2 has a more uniform, practically linear expansion curve.

After the heat treatment set forth hereinabove the product is onlyslightly sintered together. By light crushing the product can again bereduced to a fineness of up to about 70 millimicrons.

What is claimed is:

1. A process for converting monoclinic zirconium oxide to cubiczirconium oxide, which monoclinic zirconium oxide has been obtained bythe flotation separation of a zirconium oxide-silicon oxide mixture,including finely granulating the zirconium oxide recovered from saidflotation separation; treating said granulate with dilute hydrofluoricacid solution having a concentration of between 5 and 10%; separatingthe so-treated granulate from said solution, mixing said granulate withat least one oxide of a metal selected from the group consisting ofcalcium, magnesium, yttrium, thorium and cerium; and heating suchmixture to about 1,000 to 2,000 C., whereby converting said monocliniccrystal structure substantially to a cubic crystal structure in thesolid state.

2. The process claimed in claim 1 wherein said heating is from about1400 to 1900 C.

3. The process claimed in claim 1 wherein said metal oxide is one whichcrystallizes in a cubic structure.

4. The process claimed in claim 1 wherein said admixture is molded intoa shaped article prior to said heating.

5. The process claimed in claim 1 wherein said hydrofluoric acidtreating is for about 1 to 3 hours.

6. The process claimed in claim 1 wherein said metal oxide is present ina proportion of about 3 to 6 weight percent based upon the 'weight ofthe zirconium oxide.

7. The process claimed in claim 1 wherein said metal oxide is calciumoxide.

8. The process claimed in claim 1 wherein said heating is for about 4 to24 hours.

9. A process according to claim 1 wherein the monoclinic zirconium oxidebefore treatment with the hydrofluoric acid has a residual content notgreater than 0.65% silica.

References Cited UNITED STATES PATENTS 1,796,170 3/ 1931 Terwilliger42375 2,578,748 12/ 1951 Schoenlaub 423-82 2,076,080 4/ 1937 George etal. 423-75 3,514,252 5/1970 Levy et al. 2,996,369 8/1961 Harris et al.3,525,597 8/ 1 970 Mazdlyasni et al.

FOREIGN PATENTS 1,118,178 11/1967 Germany.

HERBERT T. CARTER, Primary Examiner US. Cl. X.R.

mm No. 3 .849.532 Dated umum mnuuq l'ltunn uruw. I CERTIIEICATE OFCORRECTION Novgmber 19 1 4 Invefi cm-(s) ma Deneke, Wilhelm Falkennainand Peter Hack It is certified that error appears in the aboyc-idcnti1cdpatent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 10 The priority data is missing.

Insert Priority is claimed on the basis of Germ ny Application No. P 1911 386.1 of March 6, .1969.

Signed and seal'e'd this 21st day of January 1975.

' (SEAL) Attestt' MCCOY M. GIBSON JR. C. MARSHALL DANN Attesting OfficerCommissioner of Patents

1. A PROCESS FOR CONVERING MONOCLINIC ZIRCONIUM OXIDE TO CUBIC ZIRCONIUMOXIDE, WHICH MONOLINIC ZIICONIUM OXIDE HAS BEEN OBTAINED BY THEFLOTATION SEPARATION OF A ZIRCONIUM OXIDE-SILICON OXIDE MIXTURE,INCLUDING FINELY GRANULATING THE ZIRCONIUM OXIDE RECOVERED FROM SAIDFLOTATION SEPARATION; TREATING SAID GRANULATE FROM SAID FLUORIIC ACIDSOLUTION HAVING A CONCENTRATION OF BETWEEN 5 AND 10% SEPARATING THESO-TREATED GRANULATE FROM SAID SOLUTION, MIXING SAID GRANULATE WITH ATLEAST ONE OXIDE OF A METAL SELECTED FROM THE GROUP CONSISTING OFCALCIUM, MAGNESIUM, YTTRIUM, THORIUM AND CERIUM; AND HEATING SUCHMIXTURE TO ABOUT 1,000 TO 2,000:C., WHEREBY CONVERTING SAID MONOCLINICCRYSTAL STRUCTURE SUBSTANTIALLY TO A CUBIC CRYSTAL STRUCTURE IN THESOLID STATE.